A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, to manufacture integrated circuits (ICs). In such a case, a patterning device, for example, a mask or a reticle, can generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (for example, including part of, one, or several dies) on a substrate (for example, a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. Generally, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatuses include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In the lithographic apparatus, the radiation beam may cause thermal effects (e.g., thermal expansion) in the patterning device and more generally in the patterning apparatus. The patterning device may comprise a base material, for example, fused silica, that is substantially transparent to radiation of the radiation beam, for example, deep ultraviolet radiation, and may comprise a pattern made of a substantially non-transparent material, for example, chrome. The thermal effects may be due to absorption of the radiation of the radiation beam by non-transparent portions of the patterning device and may cause, for example, alignment errors and/or overlay errors in the patterns formed on the substrate. To correct these errors due to thermal expansion of the patterning device and/or heating of gas around the patterning device, current lithographic apparatus may rely on known correction systems. For example, such a correction system can include a reticle or wafer alignment system, a magnification correction system, a feed forward system for expansion prediction, a lens correction system, or a combination thereof. However, with the continuing trend towards scaling down of device dimensions, these correction systems may not provide the desired level of alignment and/or overlay accuracy that may be needed for the development of these scaled down devices.
Additionally, radiation of the radiation beam may be incident on a part of a support structure configured to support the patterning device. The support structure may be heated due to absorption of radiation from the radiation beam. Radiation from the radiation beam may also heat the gas around the support structure which may in turn heat the support structure by convection heating. Heat may also be transferred by conduction between the patterning device and the support structure. These modes of heat transferal to or from the support structure can lead to thermal variation which may lead to thermal variation, e.g. expansion, of the support structure. Such thermal expansion can alter the position of the patterning device and may cause, for example, alignment errors and/or overlay errors in the patterns formed on the substrate.
Additionally, measurement systems may be used to align the patterning device. Measurement systems may comprise components and/or marks around the patterning device to measure the location and/or displacement of the patterning device accurately to pattern the radiation beam. The radiation beam, or transfer of heat to or from the patterning device and/or support structure may also heat the gas between the patterning device and the component and/or mark of the measurement system. Such heated gas between the patterning device and the component and/or mark of the measurement system may cause, for example, alignment errors and/or overlay errors in the patterns formed on the substrate. Known correction systems may not adequately account for the errors induced by thermal variation of components and/or marks around the patterning device used to position the patterning device.